Characteristics of the Prior Art
It has been the general understanding, heretofore, that to provide good playability in games using a racket to strike a ball such as in tennis, squash, badminton, racket ball, etc. it is desirable to utilize a string network made of strings which have a high tensile strength and a low elasticity or high elastic modulus. The general understanding has been that high deflections of the string membrane structure are undesirable in that they cause the angle of departure of the ball from the strings to vary widely over the face of the strings causing poor directional control (See, Am. J. Physics, Vol. 47, No. 6, page 484, Prof. H. Brody). With the advent of larger span racket faces, this problem became more pronounced. To reduce the problem manufacturers began to recommend use of higher initial string tensions proceeding from 45-55 pounds to 65-80 pounds. To permit these high tensions to be used higher tensile strength strings were required.
It has generally been believed, heretofore, that it was necessary to have a low elasticity or a high elastic modulus in the string to be able to maintain these high tensions over time. (See Tennis, September 1983, pg. 44, Tracy Leonard, Equipment Editor.)
It has also been generally believed that an important aspect of play is to be able to impart a controlled spin to the ball to both change its flight path and bounce to confuse the opponent. It has been believed, heretofore, that to achieve this spin it is desirable to provide an increase in friction between the ball and strings and that this can be enhanced by providing a rougher or more irregular surface to the strings which are used to make the string membrane (See U.S. Pat. No. 4,005,863; Feb. 1, 1977; Henry), (See U.S. Pat. No. 3,926,431; Dec. 16, 1975, Delorean).
Rivers in U.S. Pat. No. 4.055,941 describes a method of string manufacture which minimizes bonding material and which provides a string with a "tangent modulus" of 247,000 psi/in/in to 560,000 psi in the examples shown in his patent, where the tangent modulus is the elastic modulus of the string at the initial tensions or the string stress when the racket is not in play. The breaking stress of this string is approximately 75,000 psi. The "tangent modulus" of gut strings are typically 250K psi/in/in. River's strings are similar to gut in play, and this is deemed to be a desirable characteristic.